Cable pulling devices are well known and have been used in the construction industry to thread or draw electrical cable or wire through conduits, trays or raceways. Conventional cable pulling devices include a strong support frame, a capstan assembly mounted on the support frame and means for driving the capstan assembly. The driving means for the capstan may be a manual crank or an electric motor coupled to a chain and sprocket drive train. Typical cable pulling devices are illustrated in U.S. Pat. No. 2,221,903 issued Nov. 19, 1940 and U.S. Pat. No. 3,190,616 issued June 22, 1965.
In one known cable pulling device, the output shaft of the electric motor drives a pair of chain and sprocket drive assemblies mounted on the support frame. One of the sprockets is connected directly to the capstan for rotating same for cable pulling. This sprocket includes a plurality of radially-extending slots which are spaced apart circumferentially around the sprocket. A spring-biased pawl is mounted on the support frame of the cable pulling device and is adapted to engage the slots during rotation. The pawl is so configured that rotation of capstan sprocket may occur in one direction but not the other. In particular, when viewed from the unsupported end of the capstan, the pawl allows the sprocket to be freely rotated clockwise for cable pulling purposes and restricts rotation in the counterclockwise direction. However, this pawl and sprocket arrangement suffers from the disadvantage that some limited counterclockwise rotation of the capstan and capstan sprocket is possible by virtue of the circumferential spacing existing between the sprocket slots. For example, if the pawl has just disengaged from one of the sprocket slots and the electric motor power source is turned off or otherwise interrupted, the energy associated with the cable being pulled causes a very large counterclockwise force (torque) on the capstan and will cause the capstan to rotate counterclockwise with great speed until the pawl engages into the next circumferentially spaced sprocket slot. This counterclockwise capstan rotation causes a reverse rotation of all drive train components including the motor. When the pawl finally engages, the reverse rotation of the motor and drive train expends its kinetic energy on the components of the drive train system, frequently causing chain breakage or other damage to the drive train components. The pulling operation of course must then be interrupted to replace the chain or repair the damaged components.